Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is . LAGP belongs to the NASICON (Sodium Super Ionic Conductors) family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries. Typical values of ionic conductivity in LAGP at room temperature are in the range of 10 - 10 S/cm, even if the actual value of conductivity is strongly affected by stoichiometry, microstructure, and synthesis conditions. Compared to lithium aluminium titanium phosphate (LATP), which is another phosphate-based lithium solid conductor, the absence of titanium in LAGP improves its stability towards lithium metal. In addition, phosphate-based solid electrolytes have superior stability against moisture and oxygen compared to sulfide-based electrolytes like (LGPS) and can be handled safely in air, thus simplifying the manufacture process. Since the best performances are encountered when the stoichiometric value of x is 0.5, the acronym LAGP usually indicates the particular composition of , which is also the typically used material in battery applications.
Properties
Crystal structure Lithium-containing NASICON-type crystals are described by the general formula , in which M stands for a metal or a metalloid (Ti, Zr, Hf, Sn, Ge), and display a complex three-dimensional network of corner-sharing octahedra and phosphate tetrahedra. Lithium ions are hosted in voids in between, which can be subdivided into three kinds of sites:
Li(1) 6-fold coordinated sites at Wyckoff 6b position; Li(2) sites at Wyckoff 18e position; Li(3) sites at Wyckoff 36f position.
In order to promote lithium conductivity at sufficiently high rates, Li(1) sites should be fully occupied and Li(2) sites should be fully empty. Li(3) sites are located between Li(1) and Li(2) sites and are occupied only when large tetravalent cations are present in the structure, such as Zr, Hf, and Sn. If some Ge cations in the (LGP) structure are partially replaced by Al cations, the LAGP material is obtained with the general formula . The single-phase NASICON structure is stable with x between 0.1 and 0.6; when this limit is exceeded, a solid solution is no more possible and secondary phases tend to be formed. Although Ge and Al cations have very similar ionic radii (0.53 Å for Ge vs. 0.535 Å for Al), cationic substitution leads to compositional disorder and promotes the incorporation of a larger amount of lithium ions to achieve electrical neutrality. Additional lithium ions can be incorporated in either Li(2) or Li(3) empty sites.
In the available scientific literature, there is not a unique description of the sites available for lithium ions and of their atomic coordination, as well as of the sites directly involved during the conduction mechanism. For example, only two available sites, namely Li(1) and Li(2), are mentioned in some cases, while the Li(3) site is neither occupied nor involved in the conduction process. This results in the lack of unambiguous description of LAGP local crystal structure, especially concerning the arrangement of lithium ions and site occupancy when germanium is partially replaced by aluminium.
LAGP displays a rhombohedral unit cell with a space group R3c.
Vibrational properties
Factor group analysis LAGP crystals belong to the space group D63d - R3c. The factor group analysis of NASICON-type materials with general formula MIM2IVPO4 (where MI stands for a monovalent metal ion like Na+, Li+ or K+, and MIV represents a tetravalent cation such as Ti4+, Ge4+, Sn4+, Zr4+ or Hf4+) is usually performed assuming the separation between internal vibrational modes (i.e. modes originating in PO4 units) and external modes (i.e. modes arising from the translations of the MI and MIV cations, from PO4 translations, and from PO4 librations).
Focusing on internal modes only, the factor group analysis for R3c space group identifies 14 Raman-active modes for the PO4 units: 6 of these modes correspond to stretching vibrations and 8 to bending vibrations. +Raman-active modes in NASICON materialsVibrationFactor group Assignment ν1A1g + EgSymmetric stretchingν22A1g + 2EgSymmetric bendingν3A1g + 3EgAntisymmetric stretchingν4A1g + 3EgAntisymmetric bending
On the contrary, the analysis of external modes leads to many available vibrations: since the number of irreducible representations within the rhombohedral R3c space group is restricted, interactions among different modes could be expected and a clear assignment or discrimination becomes unfeasible.
Raman spectra The vibrational properties of LAGP could be directly probed using Raman spectroscopy. LAGP shows the Raman features characteristic of all the NASICON-type materials, most of which caused by the vibrational motions of PO4 units. The main spectral regions in a Raman spectrum of NASICON-type materials are summarized in the following table. +Spectral markers of LAGPSpectral regionAssignment< 350 cm−1Low frequencyExternal modes due to translation of the MIV cations (Ge4+ in the particular case of LAGP) or translation/libration motions of phosphate tetrahedra. 300 – 500 cm−1P – O – P symmetrical bending.500 – 800 cm−1Medium frequencyP – O – P antisymmetric bending.900 – 1300 cm−1High frequencyStretching vibrations in PO4 tetrahedra. The discrimination between symmetric and antisymmetric stretching vibration is usually not possible.
The Raman spectra of LAGP are usually characterized by broad peaks, even when the material is in its crystalline form. Indeed, both the presence of aluminium ions in place of germanium ions and the extra lithium ions introduce structural and compositional disorder in the sublattice, resulting in peak broadening.
Transport properties LAGP is a solid ionic conductor and features the two fundamental properties to be used as a solid-state electrolyte in lithium-ion batteries, namely a sufficiently high ionic conductivity and a negligible electronic conductivity. Indeed, during battery operations, LAGP should guarantee the easy and fast motion of lithium ions between cathode and anode, while preventing the transfer of electrons.
As stated in the description of the crystal structure, three kinds of sites are available for hosting lithium ions in the LAGP NASICON structure, i.e. the Li(1) sites, the Li(2) sites and the Li(3) sites. Ionic conduction occurs because of hopping of lithium ions from Li(1) to Li(2) sites or across two Li(3) sites. The bottleneck to ionic motion is represented by a triangular window delimited by three oxygen atoms between Li(1) and Li(2) sites.
The ionic conductivity in LAGP follows the usual dependency on temperature expressed by an Arrhenius-type equation, which is typical of most of solid-state ionic conductors:
where
is the pre-exponential factor, is the absolute temperature, is the activation energy for ionic transport, is the Boltzmann constant. Typical values for the activation energies of bulk LAGP materials are in the range of 0.35 - 0.41 eV. Similarly, the room-temperature ionic conductivity is closely related to the synthesis conditions and to the actual material microstructure, therefore the conductivity values reported in scientific literature span from 10 S/cm up to 10 mS/cm, the highest value close to room temperature reported up to now. Compared to LGP, the room-temperature ionic conductivity of LAGP is increased by 3-4 orders of magnitude upon partial substitution of Ge by Al. Aluminium ions have a lower charge compared to Ge ions and additional lithium is incorporated in the NASICON structure to maintain charge balance, resulting in an enlarged number of charge carriers. The beneficial effect of aluminium is maximized for x around 0.4 - 0.5; for larger Al content, the single-phase NASICON structure is not stable and secondary phases appear, mainly AlPO4, , and GeO2. Secondary phases are typically nonconductive; however, small and controlled amounts of AlPO4 exert a densification effect which affects in a positive way the overall ionic conductivity of the material.
The prefactor in the Arrhenius equation can in turn be written as a function of fundamental constants and conduction parameters:
where is ion valence, is the elementary charge, is the absolute temperature, is the Boltzmann constant, is the concentration of charge carriers, is the average velocity of the ions, is the mean free path. The prefactor is directly proportional to the concentration of mobile lithium-ion carriers, which increases with the aluminium content in the material. As a result, since the dependency of the activation energy on aluminium content is negligible, the ionic conductivity is expected to increase with increasing Ge substitution by Al, until secondary phases are formed. The introduction of aluminium also reduces the grain boundary resistivity of the material, positively impacting on the total (bulk crystal + grain boundary) ionic conductivity of the LAGP material.
As expected for solid ionic conductors, the ionic conductivity of LAGP increases with increasing temperature.
Regarding the electronic conductivity of LAGP, it should be as low as possible to prevent electrical short circuit between anode and cathode. As for ionic conductivity, the exact stoichiometry and microstructure, strongly connected to the synthesis method, have an influence on the electronic conductivity, even if the reported values are very low and close to (or lower than) 10 S/cm.
Thermal properties
The specific heat capacity of LAGP materials with general formula fits into the Maier-Kelley polynomial law in the temperature range from room temperature to 700 °C:
where
is the absolute temperature, are fitting constants.
Typical values are in the range of 0.75 - 1.5 J⋅g−1⋅K−1 in the temperature interval 25 - 100 °C. The constants increase with the x value, i.e. with both the aluminium and the lithium content, while the constant does not follow a precise trend. As a result, the specific heat capacity of LAGP is expected to increase as the Al content grows and the Ge content decreases, which is consistent with data about the relative specific heats of aluminium and germanium compounds.
In addition, the thermal diffusivity of LAGP follows a decreasing trend with increasing temperature, irrespective of the aluminium content:
The aluminium level affects the exponent , which varies from 0.08 (high Al content) to 0.11 (low Al content). Such small values suggest the presence of a large number of point defects in the material, which is highly beneficial for solid ionic conductors. Finally, the expression for the thermal conductivity can be written:
where
is the heat capacity per unit volume, is the average phonon group velocity, is the phonon mean free path, is the density of the material.
Taking everything into account, as the aluminium content in LAGP increases, the ionic conductivity increases as well, while the thermal conductivity decreases, since a larger number of lithium ions enhances the phonon scattering, thus reducing the phonon mean free path and the thermal transport in the material. Therefore, thermal and ionic transports in LAGP ceramics are not correlated: the corresponding conductivities follow opposite trends as a function of the aluminium content and are affected in a different way by temperature variations (e.g., the ionic conductivity increases by one order of magnitude upon an increase from room temperature to 100 °C, while the thermal conductivity increases by only 6%).
Thermal stability